According to PVNews in the U.S., the production of solar cell throughout the world in 2003 is 744 MW in terms of the total amount of generated power output, which increases by 12 times in the past ten years. The rapid increase of the production is due to rising concern about environmental problems, which is driven by crystal silicon solar cell occupying approximately 90% of the production of solar cell throughout the world and particularly, silicon solar cell occupying more than 60% of the production and using silicon ingot produced by a casting method, whose production is the highest today.
As the silicon solar cell, high-quality products being lower in cost and having higher conversion efficiency are required. Since the conversion efficiency and the production cost of the silicon solar cell greatly depend on silicon ingot to be used, important requirements are further quality increase and cost reduction of the silicon ingot.
The quality of the silicon ingot greatly depends on number (the area) of crystal grain boundaries, property of the grain boundaries, orientation property or defect density within the crystal grains, and so on, which constitute a factor for shortening carrier life time and carrier mobility in the solar cell to reduce the energy conversion efficiency of the solar cell. In order to improve the energy conversion efficiency of the solar cell using silicon, therefore, extensive research on the above-mentioned items and establishment of manufacturing technology are required.
The silicon ingot is generally produced by pouring a silicon melt obtained by heating and melting silicon into a mold or putting a silicon raw material in a mold, followed by heating, thereby melting the silicon raw material into a silicon melt, then cooling a bottom plate of the mold while keeping warm or heating an upper part of the mold, to give a positive temperature gradient upward from the bottom plate to the silicon melt within the mold, thereby subjecting the silicon melt to unidirectional solidification.
The silicon ingot thus obtained is processed into a silicon substrate for solar cell by generally cutting away the structure of a side surface portion and a bottom surface portion of the ingot having a large number of defects and impurities or a head of the ingot whose impurities are thickened by a solidification segregation phenomenon over a thickness of not less than several millimeters, and then slicing the ingot in the thickness direction using a multi-wire saw or the like.
FIGS. 12A and 12B are vertical sectional views showing an example of a conventional silicon casting apparatus for producing silicon ingot by the above-mentioned unidirectional solidification method disclosed in Japanese Examined Patent Publication JP04-068276B (1992).
Referring to both the figures, the silicon casting apparatus in this example comprises a furnace 21 whose bottom is opened by an opening 21a. A water-cooling chill plate 26, which can be raised and lowered, is disposed in a portion of the opening 21a of the furnace 21, and a cylindrical heat insulating body 27, which can be raised and lowered within the opening 21a, separately from the water-cooling chill plate 26 is provided in a gap between an outer peripheral edge of the water-cooling chill plate 26 and an inner peripheral surface of the opening 21a. There are provided, at an upper end of the heat insulating body 27, a mold 24 for holding a silicon melt 22 inside thereof which having a bottom plate 24a and a side plate 24b raised upward from a peripheral edge of the bottom plate 24a is disposed in a state where it can be raised and lowered together with the heat insulating body 27.
In order to produce silicon ingot using the above-mentioned silicon casting apparatus, the mold 24 having the silicon melt 22 held inside thereof is first arranged at its raised position within the furnace 21 heated to a predetermined temperature, as shown in FIG. 12A. In this case, the water-cooling chill plate 26 is lowered, and is spaced apart from a bottom surface of the mold 24.
As shown in FIG. 12B, the water-cooling chill plate 26 is then raised while being cooled with cooling water 25 passed therein, and is contact to the bottom surface of the mold 24, to cool the bottom plate 24a of the mold 24. When the mold 24, the water-cooling chill plate 26, and the heat insulating body 27 are gradually lowered, and are gradually pulled out of the furnace 21 through the opening 21a, a temperature gradient occurs in the silicon melt 22 within the mold 24 because the furnace 21 is heated to the predetermined temperature, as described above. Therefore, the silicon melt 22 is subjected to unidirectional solidification, so that silicon ingot is cast.
In the above-mentioned silicon casting apparatus, a solid-liquid interface of the silicon melt 22 exists in the vicinity of the bottom plate 24a of the mold 24, close to the water-cooling chill plate 26, in early stages of cooling, so that the solidification speed is high. As the solid-liquid interface rises by the progress of solidification, however, heat resistance caused by the thickness of a solid layer increases, so that the amount of heat removed by the water-cooling chill plate 26 decreases. As a result, the solidification speed tends to reduce. Therefore, the previous document discloses that the solidification speed is controlled by a combination of the speed at which the mold 24, the water-cooling chill plate 26, and the heat insulating body 27 are lowered and pulled out of the furnace 21 and the temperature at which the furnace 21 is heated.
In the above-mentioned silicon casting apparatus, however, the cooling capability of the water-cooling chill plate 26 is constant. Even if the speed at which the mold 24 is pulled out of the furnace 21 and the temperature at which the furnace 21 is heated are adjusted, as described above, therefore, it is difficult to subject silicon to crystal growth while keeping the temperature gradient stable within a predetermined range to solidify the silicon melt at a substantially constant speed from early stages of solidification to complete solidification. Therefore, there is a problem that the silicon ingot which has the substantially uniform in the thickness direction in the crystal grain diameter, the number of crystal grain boundaries, the property of the grain boundaries, the orientation property or the defect density within the crystal grains, and so on (it is the silicon ingot which can produce silicon substrates, which are equal in above-mentioned various kinds of characteristics, as many as possible by slicing in the thickness direction) cannot be produced with good reproducibility.
FIG. 13 is a vertical sectional view showing another example of a conventional silicon casting apparatus for producing silicon ingot by the above-mentioned unidirectional solidification method disclosed in Japanese Unexamined Patent Publication JP2002-293526A.
Referring to FIG. 13, the silicon casting apparatus in this example is so adapted that an upper chamber (furnace) 31 having a heater 34 serving as a heating mechanism and a lower chamber 32 having a cooling plate 41 cooled with cooling water 42 are defined by a barrier wall 33 composed of a heat insulating material and are connected to each other by a communication port 35 provided in the barrier wall 33, a mold 38 comprising a bottom plate 38a and a side plate 38b raised upward from a peripheral edge of the bottom plate 38a for holding a silicon melt 39 inside thereof is provided within the upper chamber 31 such that it can pass through the communication port 35 by being raised and lowered using an up-and-down machine 37.
A heat insulating material 40 for closing the communication port 35 as well as insulating between a stand 36 and the cooling plate 41 at a raised position shown in FIG. 13 and the stand 36 opposed to the cooling plate 41 for cooling (heat removing) by thermally connecting the mold 38 to the cooling plate 41 at a lowered position, which is not illustrated, are stacked in this order on the up-and-down machine 37, and the mold 38 is disposed thereon.
In order to produce silicon ingot using the above-mentioned silicon casting apparatus, the mold 38 having the silicon melt 39 held inside thereof is first arranged at a raised position within the upper chamber 31 heated to a predetermined temperature, as shown in FIG. 13.
The mold 38, the stand 36, and the heat insulating material 40 are then lowered by operating the up-and-down machine 37, to oppose the stand 36 and the cooling plate 41 to each other, thereby cooling the side of the bottom plate 38a of the mold 38. Consequently, a temperature gradient occurs in the silicon melt 39 within the mold 38 because the upper chamber 31 is heated to a predetermined temperature, as described above. Therefore, the silicon melt 39 is subjected to unidirectional solidification, so that silicon ingot is cast.
In the above-mentioned silicon casting apparatus, however, the mold 38 is lowered, as described above, at the time of cooling and solidification of the silicon melt 39 that influence the quality of the silicon ingot. Therefore, the distance between the mold 38 and the heater 34 and the amount of insertion of the mold 38 into the upper chamber 31 having the heater 34 vary, so that the entrance and exit of heat to and from the mold 38 easily vary. Particularly when the upper chamber 31 is brought into an atmosphere depressurized by inert gas such as Ar, a large part of heat from the heater 34 is transferred to the mold 38 by radiation. Thus, the distance therebetween is changed, so that the amount of heat from the heater 34 to the mold 38 greatly varies.
Even in the above-mentioned silicon casting apparatus, therefore, it is difficult to stably maintain the temperature gradient from early stages of solidification to complete solidification. Therefore, silicon ingot that is substantially uniform in the thickness direction in the crystal grain diameter, the number of crystal grain boundaries, the property of the grain boundaries, the orientation property or the defect density within the crystal grains, and so on cannot be produced with good reproducibility.
In recent years, in producing silicon ingot by the unidirectional solidification method, a unidirectional solidification and refinement method in which a nozzle is provided in a furnace, inert gas such as Ar is sprayed on a surface of a silicon melt through the nozzle, to induce agitation by heat convection in the silicon melt, and metal impurities having a low distribution coefficient are refined toward the top of an ingot while restraining thickening of the metal impurities in a solid-liquid interface, to reduce the amount of impurity elements inside of the ingot has been carried out, as described in Japanese Unexamined Patent Publication JP09-71497A (1997).
In a case where the mold is raised and lowered to and from the furnace, as in the above two examples, however, the distance between a liquid surface of the silicon melt within the mold and a front end of the nozzle is changed, and a state where inert gas stays varies. Therefore, the unidirectional solidification and refinement method, described above, cannot be smoothly and uniformly carried out.
Japanese Unexamined Patent Publication JP09-71497A (1997) discloses a silicon casting apparatus so adapted that a bottom plate of a mold is cooled through a pedestal on which the mold is placed by cooling with cooling water in a state where the pedestal is fixed to a heating furnace. The above document discloses that a temperature gradient from early stages of solidification to complete solidification can be stably maintained by restraining the amount of cooling water supplied to the pedestal in early stages of solidification of a silicon melt to restrain the amount of heat removed from the mold, and gradually increasing the amount of cooling water with the progress of solidification to increase the amount of heat removed from the mold.
However, the effect of adjusting the amount of heat removed, which is brought about by the change in the amount of cooling water, is not sufficient. According to examination made by the inventors, the amount of heat removed from the mold at a high temperature is proportional to a temperature difference between the mold and a cooling mechanism and the area in which heat exchange is carried out (the heat exchange area) and particularly, more greatly depends on the latter heat exchange area. In the above document, however, the temperature difference between the mold and the cooling mechanism is only changed by increasing and decreasing the amount of cooling water, so that the area in which heat exchange is carried out is constant. Moreover, the amount of temperature change in the cooling mechanism, which is brought about by the change in the amount of cooling water, is merely a very small change, as compared with the temperature of the silicon melt at a high temperature of not less than 1414° C.
Even if the silicon casting apparatus disclosed in the above document is used, therefore, the amount of heat removed cannot be sufficiently controlled. Therefore, it is difficult to stably maintain the temperature gradient from early stages of solidification to complete solidification, so that silicon ingot that is substantially uniform in the thickness direction in the crystal grain diameter, the number of crystal grain boundaries, the property of the grain boundaries, the orientation property or the defect density within the crystal grains, and so on cannot be produced with good reproducibility.